1. Field of the Invention
The present invention relates to a photomask blank for use in manufacturing a semiconductor integrated circuit or the like including an accurate fine pattern, and a method for manufacturing the photomask.
2. Description of the Related Art
In recent years, with increasing integration of semiconductor devices such as large-scale integrated circuits (LSI), circuit patterns have been increasingly miniaturized. For implementation of such fine circuit patterns, there has been a demand to thin interconnect patterns and contact hole patterns which form circuits. These fine circuit patterns are commonly carried out by photolithography using a photomask, thus leading to a demand to finely and accurately form the pattern of the photomask, which serves as a master.
A common method for manufacturing a photomask pattern is as follows. A resist film is formed on a photomask blank with a light blocking film formed on a transparent substrate such as a glass substrate. Thereafter, a pattern is drawn on the resist film by light or an electron beam. The resist film with the pattern drawn thereon is developed to form a resist pattern. The resist pattern is used as an etching mask to etch the light blocking film. Thus, a pattern (photomask pattern) including a light blocking portion and a light transmission portion is formed. With the increasingly miniaturized photomask pattern, the drawing is mostly based on electron beam exposure. Furthermore, the etching is mostly based on dry etching using gas plasma.
For an electron beam lithography apparatus for use in drawing patterns, every effort has been made to increase acceleration voltage in order to achieve high resolution in response to a demand for pattern miniaturization. However, a higher acceleration voltage for an electron beam increases the amount of electrons passing through the resist film when the resist is subjected to the drawing. Hence, disadvantageously, the resist may become less sensitive.
Thus, the resist for use in manufacturing a photomask is mostly of a chemical amplification type that is very sensitive. A typical chemical amplification resist uses what is called a photoacid generator that generates an acid by means of light or an electron beam. A negative resist is a resist a portion of which is subjected to a crosslinking reaction with an acid generated by irradiation with light or an electron beam and thus becomes less soluble to a solvent (developer). A positive resist is a resist that is made more soluble by an acid generated by irradiation with light or an electron beam to remove a protective group from a resin. The acid generated acts as a catalyst to cause a chain reaction by, for example, starting or promoting the next reaction. Thus, such a chemical amplification resist has the advantage of achieving a high resolution in spite of being sensitive (see, for example, Patent Document 1).
The light blocking film for the photomask blank is mostly formed of a chromium compound. A light blocking film formed of such a chromium compound is stacked on a transparent substrate or a phase-shift film formed of a silicon compound. Furthermore, a photomask blank has been proposed in which a silicon compound is used as a light blocking film and in which a hard mask or a antireflective film formed of a chromium compound or a chromium metal film is stacked on the light blocking film (see, for example, Patent Document 2).
However, in the photomask blank in which a chemical amplification resist is provided directly on the film containing chromium, the cross-sectional shape of the chemical amplification resist may be degraded at the interface between the film containing chromium and the chemical amplification film. Specifically, the chemical amplification resist is shaped like a hemming bottom when the resist is of a positive type and is shaped like a dug-in when the resist is of a negative type. This is because the acid is coordinated to a lone pair (unshared electron pair) in the chromium compound to deactivate a portion of the acid in the resist which is located near the interface.
If the cross-sectional shape of the resist is thus degraded, a deviation from a desired size occurs when the film containing chromium is etched using the resist pattern as an etching mask. Specifically, with the positive resist, the hemming bottom shape prevents micro spaces or holes from being properly formed. Furthermore, with the negative resist, the dug-in shape causes the resist pattern to be collapsed during development or cleaning.
As means for solving these problems, a method for stacking a film formed of a silicon compound containing no chromium, on a film containing chromium (see, for example, Patent Document 3). However, the film containing chromium is not substantially etched by dry etching using fluorine (F) but by dry etching using oxygen-containing chlorine (Cl/O). In contrast, the film formed of a silicon compound containing no chromium is not substantially etched by the dry etching using oxygen-containing chlorine (Cl/O) but by the dry etching using fluorine (F). This increases the numbers of steps and conditions for the dry etching and thus a cycle time for production of a photomask. Furthermore, this method is disadvantageous for rate control for an etching apparatus and defect management. Additionally, the dry etching using oxygen-containing chlorine (Cl/O) is less directional and results in isotropic etching. Hence, the size of a film containing chromium which has been patterned by the dry etching using oxygen-containing chlorine deviate from that of the film formed of a silicon compound containing no chromium and which has been patterned by the dry etching using fluorine (F), which is anisotropic. This causes an error in the size of the final photomask pattern. Furthermore, if the two films with the different sizes are used as an etching mask to further etch an underlying film, the cross-sectional shape of the underlying film may disadvantageously be degraded.